1. Field of the Invention
The present invention relates to recovery of elemental sulfur from gas streams containing sulfur dioxide.
2. Description of the Prior Art
Flue gases emitted from burning sulfur-containing fossil fuels are the most common dilute sulfur dioxide (SO.sub.2) containing industrial gases. The majority of commercial scale flue gas desulfurization (FGD) plants in use today for combustion gas purification are based on wet scrubbing processes. Many of them are of the "throwaway" type, fixing the sulfur in a solid waste product, which requires disposal. These FGD systems do not recover elemental sulfur. However, several other wet and dry FGD processes are of the regenerative type combining sulfur dioxide removal with active medium regeneration and concomitant sulfur recovery.
Many sulfur recovery methods have been proposed. Often the type and location of the primary operation (sulfur source) dictate the choice of the sulfur recovery method. For example, sulfur recovery from metallurgical operations (smelters, sulfide ore roasters) is typically in the form of sulfuric acid. On the other hand, petroleum refineries emit H.sub.2 S-rich gas streams which are processed in multi-stage Claus plants to recover elemental sulfur.
Recovery of sulfur values in elemental sulfur form is more desirable than sulfuric acid or liquid SO.sub.2 as local market conditions are typically more restrictive for the latter. (See, for example, J. B. Rinckhoff, J. B. Pfeiffer, (ed.), "Sulfur Removal and Recovery from Industrial Processes," Advances in Chemistry Series, No. 139, p. 48, American Chemical Society, 1975). For SO.sub.2 -containing industrial gases, this means reducing the SO.sub.2 with a gaseous reducing agent, such as carbon monoxide, hydrogen, synthesis gas (H.sub.2 +CO), or natural gas, or with a carbonaceous solid (such as activated charcoal, coke, anthracite coal). The Allied Chemical sulfur dioxide reduction technology employs a catalyst over which SO.sub.2 reduction by natural gas (CH.sub.4) takes place, producing a mixture of H.sub.2 S, elemental sulfur and (unconverted) SO.sub.2. After condensation of sulfur, further sulfur recovery is accomplished in two-stage Claus plants. This process requires relatively concentrated SO.sub.2 (&gt;4.0%) gases and downstream Claus plants to complete sulfur recovery. (See, for example, W. D. Hunter, Jr., "Reducing SO.sub.2 in Stack Gas to Elemental Sulfur," Power 117 (9), 63, 1973; Watson et al., U.S. Pat. No. 3,653,833; and Bridwell et al., U.S. Pat. No. 3,755,551.)
The RESOX process, developed by the Foster Wheeler Energy Corporation, and described in "The FW-BF SO.sub.2 Removal System," Sulfur, No. 119, 24-26 and 45, July-August 1975, partially reduces the SO.sub.2 -rich streams (&gt;10.0% SO.sub.2) to elemental sulfur and organosulfur compounds by reaction with coke at 850.degree.-900.degree. C. (See also, R. E. Rush, and R. A. Edwards, "Operating Experience with Three 20 MW Prototype Flue Gas Desulfurization Processes at Gulf Power Company's Scholtz Electric Generating Station," presented at EPA Flue Gas Desulfurization Symposium, Hollywood, Fla., November 8-11, 1977).
Direct flue gas reduction by synthesis gas over an undisclosed catalyst is proposed by K. V. Kwong et al. in "The Parsons FGC Process Simultaneous Removal of SO.sub.x and NO.sub.x'," presented at the 1990 Annual Meeting of AIChE, Chicago, Ill., Nov. 11-16, 1990, to simultaneously reduce the oxygen, SO.sub.x and NO.sub.x in the flue gas. The H.sub.2 S produced is selectively recovered by solvents, concentrated and taken to multi-stage Claus plants for elemental sulfur recovery. This process does not achieve a single-step SO.sub.2 reduction to sulfur. Similarly, earlier proposed schemes of flue gas reduction could not achieve both high SO.sub.2 conversion as well as high selectivity to elemental sulfur in a single-stage catalytic reactor.
In addition to power plant SO.sub.2 emissions, dilute sulfur dioxide-containing gas streams are produced in waste incinerators, industrial furnaces, process equipment used in petroleum refineries and sulfuric acid plants, and spent sorbent or catalyst regenerator equipment. Sulfur recovery involves several steps, such as partial reduction of SO.sub.2 to H.sub.2 S, followed by Claus processing. No single-stage process presently exists to directly reduce the varying SO.sub.2 -effluent gases to elemental sulfur over a catalyst which displays both high activity and high selectivity.
The catalytic removal of sulfur dioxide by carbon monoxide involves a main reaction producing elemental sulfur: ##EQU1## where x varies between 2 and 8, as well as a competing side reaction producing carbonyl sulfide: EQU CO+S=COS (2)
At about the stoichiometric ratio of CO/SO.sub.2 reaction (1) is favored, while excess CO increases production of COS.
P. R. Ryason et al. in U.S. Pat. No. 3,454,355 (and Air Pollut. Contr. Ass. 17, 796, 1967), reported on the use of single-bed catalysts (Cu, Pd, Ag, Co or Ni supported on alumina) to produce sulfur from dry sulfur dioxide gases. L. A. Haas et al., in U.S. Pat. No. 3,888,970, employed a double layer catalyst bed for the reduction of SO.sub.2 by CO to elemental sulfur with alumina-supported Fe, Cr, Ni, Mn, or Co as first layer and alumina as second layer. A. B. Stiles, in U.S. Pat. No. 3,856,459, proposed the reduction of SO.sub.2 by a refractory reducing gas over a supported catalyst containing thorium oxide in combination with one or more oxides of Cr, Mn, Ba, Sr, Ca, Ta or mixed rare earth chromites. This process produced a gas mixture of elemental sulfur, hydrogen sulfide and other sulfur containing compounds. For the selective catalytic reduction of SO.sub.2 by CO to elemental sulfur under dry conditions, L. Bajars proposed mixed oxides of elements from the lanthanide group and the groups IVB and VB of the Periodic Table in U.S. Pat. No. 3,978,200. This type of catalyst typically needs to be activated by reducing gases at high temperature. J. M. Whelan, in U.S. Pat. No. 4,081,519, disclosed a ceramic catalyst of the following composition for the oxidation of CO by SO.sub.2 : W.sub.k X.sub.n J(.sub.1-k-n) ZO.sub.(3.+-.m)), wherein W is Zr, Sn, Th or mixture thereof, X is an alkaline earth metal or mixture thereof, J is Sc, Y, a rare-earth element or mixture thereof, and Z is a metal of the first transition series or a mixture thereof.
Perovskite-type mixed oxides (ABO.sub.3) containing transition metals have long attracted attention as catalysts for heterogeneous reduction/oxidation reactions. (See, for example, D. B. Meadowcroft, Nature, 226, 847, 1970; and R. J. H. Voorhoeve, et al., Science, 177, 353, 1972). Generally, these type of oxides possess high electronic and oxygen ionic mobility, and a variety of surface sites for the adsorption/description of reacting species. These are essential properties required for a reduction/oxidation reaction. Therefore, perovskite oxides have been widely investigated for the reactions of carbon monoxide, nitric oxide and hydrocarbons in the field of environmental pollution control. However, most of these reactions are poisoned by sulfur dioxide. Recently, the strontium substituted lanthanum cobalt perovskite oxide (La.sub.1-x Sr.sub.x CoO.sub.3), a well known perovskite catalyst, has been extensively studied as a catalyst for the reduction of SO.sub.2 by CO by D. B. Hibbert and R. H. Campbell in Applied Catalysis 41, 173 and 289, 1988. Nearly all SO.sub.2 was converted to elemental sulfur with a dry feed gas of stoichiometric SO.sub.2 and CO composition over the catalyst, when x=0.3. However, this catalyst lost the perovskite structure and became a mixture of sulfides and oxysulfides of the metals after a short induction period under reaction conditions.
The prior art, therefore, does not teach or suggest the formulation of an active and stable catalyst for direct elemental sulfur recovery from SO.sub.2 -containing industrial gas streams by reacting these gas streams with a reducing gas, particularly, in the presence of water vapor.